Precise coupling of spatially separated intracellular ATP-producing and ATP-consuming processes is fundamental to the bioenergetics of living organisms. Integrated spatially arranged phosphotransfer systems catalyzed by creatine kinase, adenylate kinase, carbonic anhydrase, and glycolytic enzymes provide efficient high-energy phosphoryl transfer to support cellular functions and maintain intracellular energy homeostasis under stress (see, e.g., Dzeja and Terzic, J Experimental Biol 2003, 206, 2039-2047). Creatine kinase catalyzes the reversible transfer of the N-phosphoryl group from phosphocreatine to ADP to regenerate ATP and plays a key role in the energy homeostasis of cells with intermittently high, fluctuating energy requirements such as skeletal and cardiac muscle, neurons, photoreceptors, spermatozoa, and electrocytes. The creatine kinase system has a dual role in intracellular energy metabolism—functioning as an energy buffer to restore depleted ATP levels at sites of high ATP hydrolysis, and to transferring energy in the form of phosphocreatine from the mitochondria to other parts of the cell by a process involving intermediate energy carriers, several enzymatic reactions, and diffusion through various intracellular structures.
Many pathological disease states arise from a dysfunction in energy metabolism. Cellular depletion of ATP stores, as occurs for example during tissue ischemia, results in impaired tissue function and cell death. Of foremost medical relevance, ischemia-related cardiovascular disease such as stroke and heart attack remains a leading cause of death and morbidity in North America and Europe. Thus, strategies that can prevent or reverse ischemia-related tissue damage are expected to have a major impact on public health. Energy depletion also contributes to tissue damage during surgery and is a common cause of organ transplant failure. Furthermore, reperfusion with oxygen-containing solutions can further exacerbate tissue health through production of oxygen radicals. Therefore, a method to rapidly restore ATP levels without causing reperfusion injury is likely to have many therapeutic applications. Neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and Huntington's disease are associated with impaired energy metabolism, and strategies for improving ATP metabolism could potentially minimize loss of neurons and thereby improve the prognosis of patients with these diseases. Finally, impaired energy metabolism is an important factor in muscle fatigue and limits physical endurance. Therefore, a method of preventing or reversing ATP depletion in ischemic or metabolically active tissues is likely to have broad clinical utility in a wide range of indications.
A large body of research indicates that the loss of cellular ATP due to oxygen and glucose deprivation during ischemia is a cause of tissue death. To prevent this, mammalian cells harbor protective biochemical mechanisms for minimizing ATP depletion during ischemia and episodes of high metabolic demand as occurs in metabolically active brain or muscle tissues. The creatine kinase system is a key biochemical mechanism that prevents ATP depletion in mammalian cells. Phosphagens such as creatine phosphate (4):
are high-energy phosphate sources that can regenerate ATP when intracellular levels of ATP fall. The level of creatine phosphate in a cell is an important predictor of resistance to ischemic insult, and remaining stores of creatine phosphate are correlated with the extent of tissue damage. Studies have documented the importance of creatine phosphate levels in cardiac and brain ischemia, neuronal degeneration, organ transplant viability, and muscle fatigue (see, e.g., Wyss and Kaddurah-Daouk, Physiological Reviews 2000, 80(3), 1107-1213, which is incorporated by reference herein in its entirety). Accordingly, the administration of creatine or creatine phosphate for treating these and other diseases is being explored (see, e.g., Kaddurah-Daouk et al., U.S. Application Publication Nos. 2005/0256134, and 2003/0018082, and U.S. Pat. No. 6,075,031 (use of creatine kinase analogs for treating glucose metabolic disorders); Kaddurah-Daouk, U.S. Application Publication No. 2004/0116390, and U.S. Pat. No. 5,998,457 (obesity and related disorders), Kaddurah-Daouk, U.S. Application Publication No. 2004/0054006 (transmissible spongiform encephalopathies); Kaddurah-Daouk et al., U.S. Application Publication Nos. 2004/0102419, 2004/0106680, and 2002/0161049, and U.S. Pat. No. 6,706,764 (diseases of the central nervous system); and Lambert et al., Adv Phys Med Rehab, 2003, 84(8), 1206-1210 (multiple sclerosis).
Creatine, (5),
supplementation increases intracellular creatine phosphate levels (Harris et al., Clinical Sci 1992, 83, 367-74). Creatine phosphate (2 gm/day) given to athletes during strenuous endurance training has allowed the athletes to train longer with less muscle stiffness. Because creatine phosphate is readily metabolized when administered orally it must be administered intramuscularly or intravenously to be effective. Creatine easily crosses the blood-brain barrier and brain creatine levels can be increased via oral administration (Dechent et al., Am J Physiol 1999, 277, R698-704). Prolonged creatine supplementation can elevate the cellular pools of creatine phosphate and increase resistance to tissue ischemia and muscle fatigue. However, creatine supplementation typically takes weeks to increase creatine phosphate levels, and the overall increase is generally fairly small (<50%). For example, human studies show that in healthy volunteers cerebral creatine phosphate can be increased only by about 10% by oral creatine administration (Dechent et al., Am J Physiol 1999, 277, R698-R704). Interestingly, increases in tissue creatine phosphate levels following oral creatine supplementation are long-lasting (>14 days), suggesting that strategies that increase creatine phosphate could have long lasting beneficial effects and would be effective with infrequent dosing. However, acute application of creatine is not effective in restoring tissue ATP levels, and therefore may have limited value in emergency care situations. Application of creatine phosphate to cells does not raise intracellular creatine phosphate, since due to its high polarity (hydrophilicity), creatine phosphate is not taken up into cells and does not readily cross barrier tissues such as the blood-brain-barrier. Creatine phosphate is also rapidly metabolized in biological fluids. Conjugating creatine phosphate with a protein moiety has been proposed as a strategy for enhancing translocation through barrier tissue (see, e.g., Kaddurah-Daouk et al., U.S. Application Publication No. 2004-0126366). Thus, although administration of creatine phosphate may have some therapeutic usefulness, a modified creatine phosphate molecule that is more stable and is more permeable to barrier tissues and cellular membranes would have enhanced therapeutic value.
Creatine phosphate analog prodrugs provided by the present disclosure are designed to be stable in biological fluids, to enter cells by either passive diffusion or active transport, and to release the corresponding creatine phosphate analog into the cellular cytoplasm. Such prodrugs can also cross important barrier tissues such as the intestinal mucosa, the blood-brain barrier, and the blood-placental barrier. Because of the ability to pass through biological membranes, creatine phosphate analog prodrugs can restore and maintain energy homeostasis in ATP depleted cells via the creatine kinase system, and rapidly restore ATP levels to protect tissues from further ischemic stress. Prodrugs of creatine phosphate analogs having a higher free energy, e.g., cyclocreatine, or lower affinity for creatine kinase, and which can regenerate ATP under more severe conditions of energy depletion are also disclosed. Creatine phosphate analog prodrugs provided by the present disclosure can also be used to deliver sustained systemic concentrations of the corresponding creatine phosphate analog.